This invention relates generally to bonding systems and more particularly concerns a method in which two components are bonded together using gold without other intermediate bonding materials.
Acousto-optic Bragg cells are manufactured by bonding a piezo electric substrate to an optical substrate. Cold welding systems have been used but the cold welding process is limited both in types of metals that may be used for bonding and in possible yields of working parts. For instance, indium is suited to the cold welding process, but when using indium the cold welding process must be carried out in a vacuum. In the cold welding process, the parts being cold welded together may delaminate if the parts are not sufficiently planarized with respect to each other.
Other processes which use ultrasonic welding have also been described. Ultrasonic welding allows a greater variety of metals to be used and welding pressures to be reduced by introducing heat and ultrasonic energy to the welding process. A description of ultrasonic welding apparatus and techniques is contained in Ultrasonically Welded Piezoelectric Transducers by John D. Larson, III and D. K. Winslow published in IEEE Transactions on Sonics and Ultrasonics, volume su-18, number 3, July 1971. The process, as described by Larson et. al., is to first coat the substrates with a flash of chromium or titanium to insure good adhesion of subsequent metals. Then the substrates are coated with the bonding metal and placed, with bonding surfaces in contact with each other, inside a heater. Once the heater has obtained the desired temperature, a static pressure and acoustic waves are applied to the substrates until bonding is completed.
While this system is an improvement upon traditional cold welding systems, excessive substrate heating and inadequate planarization of the substrates can cause improper bonding and breakage of parts.
Other attempts to achieve gold to gold bonds have been unsuccesful because the pressures to achieve bonding cause breakage of parts. Reduction of bonding pressure causes improper bonding and lack of adhesion.
The present process provides an alternative to both traditional cold welding process and traditional ultrasonic welding processes. This process uses a combination of interferometric planarization feedback, thermal agitation and profile heating to optimize the bonding process. This process produces better bonding characteristics with higher yields then previously known processes.
Further advantages of the invention will become apparent as the following description proceeds.